Square isn't cheap. If Final Fantasy Spirits Within isn't a testament to that then I don't know what is. The point is, when making FFX they probably didn't intend to make the storyline of FFX-2 as someone mentioned before, but following the success of the game, they decided to milk it. Thus creating a sequel that purposely connected the two characters Shuyin and Tidus. This was not accidental. They're related in that game. If you are solely going by the first game as I said, then Tidus would just be merely a dream of The Fayth. Whereas in the second game it's a bit deeper than that. It's a creation manifested by their yearning to keep a part of Zanarkand alive that had died in a more literal sense.